Montana geologist finds new Bitterroot fault line

By Eve Brown Ravalli Republic

HAMILTON, Mont. (AP) – A mapping method used to look at floodplains in the Bitterroot Valley turned up something unexpected: a new fault line that just might cross under the Lake Como Dam.

Jeff Lonn, a geologist with the Montana Bureau of Mines and Geology in Butte, said Ravalli County was using Light Detection and Ranging—or LIDAR—equipment as part of a floodplain study. LIDAR uses laser to map the ground, showing what the land is like devoid of homes, grasses and trees.

Mike Stickney, a senior research geologist and director of earthquake studies at the Bureau of Mines was watching a presentation on the LIDAR study, and realized he was seeing something that looked like a “fault scarp” in the Bitterroot.

“It came as a real surprise to me to find out there was movement on a fault, because in that part of the state the national maps show it as a low seismic area,” Stickney said. “The Bitterroot fault has long been classified by the U.S. Geological Survey as a possible young fault, but there’s not much detail known on it. But perhaps part of the reason it’s listed as such is the information is incomplete.”

A fault scarp is a steep slope that separates what previously was a relatively level area, and can be caused by earthquakes.

With tall grasses covering a landscape that’s dotted with trees, it’s easy for the untrained eye to miss the scarp. But a closer look shows where an earthquake offset the surface of the earth, dipping the east side down and pushing the west side up. In other areas, the scarp is closer to a “couple hundred feet,” Lonn said.

“It’s like when you’ve stacked blocks and you’re pushing them together. When you relieve that pressure, some of the rocks will fall down,” Lonn said. “It’s the same as what happened 50 million years ago, with the scarp that took the Bitterroot Mountains out from underneath the Sapphires.”

This scarp appears to travel along the lower east face of the Bitterroots, quite possibly through the Lake Como Dam and at least up to Victor, according to the LIDAR images. Stickney said it’s difficult to know if it goes through the dam due to glacial deposits that cover part of the scarp.

“You lose the details; it becomes foggy closer to the lake,” Stickney said. “But the general trend is toward the dam.

“This is why LIDAR is such a powerful tool. An analogy I use is that it provides a view of the ground after an apocalyptic forest fire. It’s just the bare ground.”

This is known as a “young” fault as far as geologic time is concerned. Lonn notes that the earthquake activity that separated the Bitterroots and Sapphires occurred about 50 million years ago. Since this newly found scarp is mixed with glacial deposits, it’s thought to have occurred around the end of the ice age that occurred 10,000 to 12,000 years ago.

“While that sounds like a long time ago—and it is from the human perspective—in geological time that was yesterday,” Stickney said. “The fact that it has been young movement along that fault comes as a surprise.”

Lonn said the earthquake that created the scarp probably was “cataclysmic.” Neither he nor Stickney know if, or when, the fault might produce another earthquake. Both added, though, that while the fault is loosely related to recent earthquake “swarms” in Idaho and the earthquake earlier this summer in Lincoln because of the earth’s tectonic plates, movements elsewhere probably don’t have anything to do with the Bitterroot fault.

Lonn added that more research is needed to learn whether this is an active fault.

“If the recurrence rate is one in 7,000 years, I would say our chances of seeing an earthquake are not good,” Lonn said. “But if the recurrence rate is one every 500 years…I don’t want to be alarmist, but it’s interesting that where we live is still geologically active. It may or may not be a hazard.”

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